Fable Legends will support up to four players and those four players can be in any combination of Xbox One and PC players. In addition streaming is supported on Windows 10 PCs and tablets which means that a PC player who owns the game on Xbox One can stream it to his PC and still play in a multi-player game.
The streaming of Fable Legends to the PC seems to be lag free which was good. It looks like the possibility of more games for Windows 10 are coming in the future.